For over a century, the so-called "valve" metals (i.e. metals which form adherent, electrically insulating anodic oxide films, such as aluminum, tantalum, niobium, titanium, zirconium, silicon, etc.) have been employed for film applications. These applications include electrolytic capacitors, rectifiers, lightning arrestors, and devices in which the anodic film takes the place of traditional electrical insulation, such as special transformers, motors, relays, etc.
When biased positive in appropriate (i.e. non-corrosive) aqueous or partially aqueous electrolytes, typical valve metals, such as aluminum or tantalum become coated with a dielectric film of uniform thickness. At constant temperature, the film thickness is proportional to the applied voltage and the rate of film growth is directly proportional to the current density. These properties are described at length in L. Young's book, "Anodic Oxide Films" (1961, Academic Press, London).
Additionally, the thickness of anodic films at constant voltage is directly proportional to the absolute (Kelvin) temperature of the electrolyte. This was demonstrated by A. F. Torrisi ("Relation of Color to Certain Characteristics of Anodic Tantalum Films", Journal of the Electrochemical Society Vol. 102, No. 4, April, 1955, pages 176-180) for films on tantalum over the temperature range of 0.degree. C. to 200.degree. C. and with applied voltages up to 500 volts, presumably with the glycol-borate electrolytes in use at the time (these electrolytes always contain some free water, produced by esterification, which supplies oxygen for film formation).
The above relationships of voltage, temperature, current density and anodic film thickness have been successfully exploited by the manufacturers of electrolytic capacitors to obtain anodic films of different thickness according to the finished device voltage and capacitance requirements.
Anode foil for aluminum capacitors is usually anodized, following suitable etching processes to increase surface area, by slowly passing the foil through a series of anodizing tanks, each biased progressively more negative vs. the aluminum foil. The slow rate of transit of the foil through each tank allows the anodic film to reach the limiting thickness for the voltage difference between the foil and each tank of electrolyte.
In the manufacture of tantalum capacitors, powder metallurgy techniques are used to produce slug-like capacitor bodies of significantly less than theoretical density and having high internal surface area. The anodic dielectric film is produced by immersing the capacitor bodies in an electrolyte and applying current (usually a constant current) until the desired voltage is reached and then holding the anode bodies at this voltage for a time sufficiently long to insure a uniform film thickness within the interstices of the anode bodies.
Upon application of suitable cathode contacts, anode materials covered with anodic films as described above, become positive capacitor "plates" in polar capacitors in which the anodic film serves as the dielectric. These devices are characterized by a relatively high capacitance per unit volume and relatively low cost per unit of capacitance compared with electrostatic capacitors.
These devices are also "polar" devices, which show so-called "valve" action, blocking current within the rated voltage range when the valve metal is positively biased and readily passing current if the valve metal is biased negative (early rectifiers were based upon this fact and contained aluminum or tantalum as the valve metal).
It is readily apparent that modifications of the anodizing process resulting in anodic oxide films having high dielectric constant and low film thickness per volt are advantageous as they tend to maximize capacitance per surface area of valve metal at a given anodizing voltage. C. Crevecoeur and H. J. DeWit, in a paper entitled: "The Influence of Crystalline Alumina on the Anodization of Aluminum" (Presented at the Electrochemical Society Meeting in Seattle, Wash., May 21-26, 1978) report that aluminum anodized in very dilute citric acid solutions gives rise to a "crystalline" anodic oxide with a thickness of 8 angstroms per volt, while the film produced in traditional dilute borate electrolytes has a thickness of 11 angstroms per volt. This results in an approximate 30% capacitance advantage for the films produced in the carboxylic acid solution.
The dielectric properties (i.e. withstanding voltage, dielectric constant) of the anodic film appear to be influenced to an extraordinary degree by the presence of even a small amount of carbonaceous material incorporated during anodizing.
U.S. Pat. No. 4,159,927 indicates that anodizing electrolytes containing small quantities of hydroxy-carboxylic acids (e.g. tartaric acid, malic acid, citric acid, etc.) in addition to the major boric acid solute give rise to anodic films on aluminum containing less than 1% carbon, but having profoundly different diffusion properties as indicated by their much lower rate of reaction with water to form hydrated species compared with traditional films containing no carbonaceous species. In aqueous electrolytes containing minor amounts of hydroxy-carboxylic acids, the incorporated carbonaceous species originates with the carboxylic acid carbon. This is not necessarily true for all electrolytes, however.
Solutions of boric acid in formamide give rise to anodic films on aluminum at 60-100.degree. C. which contain a significant amount of incorporated carbonaceous species ("Properties and Mechanism of Formation of Thick Anodic Oxide Films on Aluminum from the Non-Aqueous System Boric Acid-Formamide", S. Tajima, N. Baba, and T. Mori, ElectroChemical Acta, 1964, Vol. 9, pages 1509 to 1519).
GB 2,168,383A describes an anodizing process employing aprotic polar solvent solutions of phosphoric acid or soluble amine phosphate, operated below about 30.degree. C. Anodic films formed on titanium coupons in these electrolytes have been demonstrated to contain incorporated carbonaceous material. ("Anodizing Mechanism in High Purity Titanium", H. W. Rosenberg, M. S. Cooper, and Karl Bloss; presented at the "Titanium '92" 7th International Conference on Titanium, San Diego, Calif., 1992).
More recently, Ue, et al. have demonstrated that anodic films on aluminum anodized in anhydrous (about 10 ppm water) 4-butyrolactone containing quaternary ammonium salts exhibit a dielectric constant enhancement of as much as 10 to 20 times higher than that obtained with traditional aqueous anodizing electrolytes (Japanese Patent No. 8-134693). These authors have extended this anodizing method to include anhydrous solutions of quaternary ammonium salts of oxygen-containing mineral acids in ethylene glycol and have obtained a similar, though less pronounced elevation of the dielectric constant of anode films on aluminum (Japanese Patent No. 8-134,692). These authors have also claimed in the technical paper, "Anodic Oxidation of Valve Metals in Non-Aqueous Electrolyte Solutions", (Electrochemical Society Proceedings, Vol. 96-18, pages 84-95) to have extended this anodizing method to titanium, zirconium, hafnium, niobium, and tantalum, but give no supporting data for this claim. The anodic film growth in the electrolytes of Ue, et al. is traditional so far as the anodizing kinetics are concerned, with the film growing to a thickness dependent upon voltage.
The elevated dielectric constant of anodic films grown on titanium in low water content phosphate solutions in 4-butyrolactone was disclosed in GB 2,168,383A, in example no. 4, in which a dielectric constant of 8 times that of traditionally formed tantalum oxide was produced at 100 volts. In a further preferred embodiment, disclosed in example No. 7, anodic titanium oxide produced at 500 volts in a low water content phosphate solution in N-methyl-2-pyrrolidone gave a capacitance of over 30 times that of a equal surface area of tantalum anodized to 500 volts in a traditional electrolyte.
Unfortunately, all of the above anodizing methods which give rise to an elevation of the dielectric constant of the anodic oxide have major drawbacks or limitations when used in a production scale anodizing process. Quaternary ammonium salts are expensive and difficult to obtain. Amines, such as pyridine and the picolines, which form electrolyte-soluble phosphate salts tend to be toxic and to have very unpleasant odors. Many of the most suitable solvents, such as 4-butyrolactone, N-alkyl-2-pyrrolidones, dimethyl formamide, dimethyl sulfoxide, etc., are toxic, flammable or are difficult to contain in standard anodizing equipment due to attack of circulation pump seals, etc.
Furthermore, it is very difficult to maintain polar solvent-based electrolytes in an anhydrous condition in a production environment. The reduction in anodic film breakdown voltage and anodizing efficiency for aprotic solvent phosphate solutions containing more than about 2% water are described in GB 2,168,383A, while Ue, et al. describes a factor of three difference in oxide thickness per volt with a 300 ppm increase in electrolyte water content (Electrochemical Society Proceedings paper cited earlier, page 86).
The expedient of simply heating the anodizing electrolytes to temperatures above the boiling point of water to drive off moisture is impractical due to excessive solvent evaporation, increased possibility of fires, loss of volatile amines, and reaction of the solvents with the solutes. At higher temperatures, 4-butyrolactone reacts with amines and phosphates, dimethyl sulfoxide is converted into dimethyl sulfide and dimethyl sulfone and alkyl amides react with phosphates to form phosphoramides, etc.
The simple expedient of employing the methods and solvents, etc., of GB 2168,383A and replacing the phosphoric acid with polyphosphoric acid to reduce the water content has been attempted (U.S. Pat. No. 5,211,832) and, unfortunately, has been found to lead to the production of anodic titanium dioxide films having a dielectric constant of about 20. This value is several times less than that obtained with phosphoric acid according to GB 2,168,383A.
It is desired to provide an anodizing electrolyte or series of electrolytes which have the ability to produce anodic films having high dielectric constant and few flaws. It is also desired to have high thermal stability so that the water content can be maintained at sufficiently low levels with the aid of heat alone (i.e., no need for vacuum-treatment, etc.). In addition it is desired to have safe, low-toxicity, low-objectionable odor components and a near-neutral pH (i.e. a "worker-friendly" composition) and low-cost components (to make mass production affordable). Also desired is inherent stability of composition over the operating life so as to avoid the need for frequent analysis and component additions to maintain the electrolyte composition and relatively low resistivity so as to produce anodic films of uniform thickness with varying separation between anode and cathode surfaces.